Among known conventional synchronous electric motors are: a permanent magnet synchronous electric motor (PMSM), which includes a permanent magnet in a rotor; a synchronous electric motor which includes field coils in a rotor (FCSM: Field Coil Synchronous Motor); and a reluctance motor (RN), which includes magnetic salient poles in a rotor.
A PMSM has no power loss in magnetic creation and is high in efficiency. However, field-weakening control needs to be performed at the time of high-speed rotation. Moreover, there are problems related to the mechanical strength and thermal durability of the permanent magnet
An RM, or a motor without any permanent magnet, has gained attention because of the problems associated with the PMSMs and a surge in the prices of magnets. As the RM, a synchronous reluctance motor (SynRM), which rotates in synchronization with a sine wave rotating field, a switched reluctance motor (SRM), which is rotated by a switching field, and the like have been known. The SynRM is known to be lower in noise and vibration than the SRM.
As for a rotor of the SynRM, the direction of a main magnetic flux is referred to as q-axis, and a direction that is electrically and magnetically orthogonal to that direction is referred to as d-axis. The rotor of the SynRM has, for example, five layers of flux barriers (slits) within a region between two q-axes whose electrical angles are, for example, π (180 degrees) apart each other.
Accordingly, the d-axis direction inductance of the rotor, or d-axis inductance Ld, is smaller than q-axis direction inductance, or q-axis inductance Lq. As a result, the reluctance torque (=(Ld−Lq)·Id·Iq) can be increased. Here, Id is a current that creates a d-axis direction magnetic flux, and Iq is a current that creates a q-axis direction magnetic flux.
In another example of the configuration of the rotor of the SynRM, concave grooves are provided near a d-axis on an outer peripheral surface of a rotor core, and a rotor coil is wound around the concave grooves. As DC current flows through the rotor coil, the torque is generated by current flux Φi in addition to the reluctance torque.
The concave grooves are provided near the d-axis, which is defined as described above. Therefore, it is possible to prevent a decrease in the reluctance torque. As a result, it is possible to improve the torque and the efficiency.